CONTAMINATION OF MILK.
No more important lesson is to be learned than that which relates to the
ways in which milk is contaminated with germ life of various kinds; for
if these sources of infection are thoroughly recognized they can in
large measure be prevented, and so the troubles which they engender
overcome. Various organisms find in milk a congenial field for
development. Yeasts and some fungi are capable of growth, but more
particularly the bacteria.
~Milk a suitable bacterial food.~ The readiness with which milk undergoes
fermentative changes indicates that it is well adapted to nourish
bacterial life. Not only does it contain all the necessary nutritive
substances but they are diluted in proper proportions so as to render
them available for bacterial as well as mammalian life.
Of the nitrogenous compounds, the albumen is in readily assimilable
form. The casein, being insoluble, is not directly available, until it
is acted upon by proteid-dissolving enzyms like trypsin which may be
secreted by bacteria. The fat is relatively resistant to change,
although a few forms are capable of decomposing it. Milk sugar, however,
is an admirable food for many species, acids and sometimes gases being
generally produced.
~Condition when secreted.~ When examined under normal conditions milk
always reveals bacterial life, yet in the secreting cells of the udder
of a healthy cow germ life is not found. Only when the gland is diseased
are bacteria found in any abundance. In the passage of the milk from
the secreting cells to the outside it receives its first infection, so
that when drawn from the animal it generally contains a considerable
number of organisms.
[Illustration: FIG. 5. Microscopic appearance of milk showing relative
size of fat globules and bacteria.]
~Contamination of milk.~ From this time until it is consumed in one form
or another, it is continually subjected to contamination. The major part
of this infection occurs while the milk is on the farm and the degree of
care which is exercised while the product is in the hands of the milk
producer is the determining factor in the course of bacterial changes
involved. This of course does not exclude the possibility of
contamination in the factory, but usually milk is so thoroughly seeded
by the time it reaches the factory that the infection which occurs here
plays a relatively minor role to that which happens earlier. The great
majority of the organisms in milk are in no wise dangerous to health,
but many species are capable of producing various fermentative changes
that injure the quality of the product for butter or cheese. To be able
to control abnormal changes of an undesirable character one must know
the sources of infection which permit of the introduction of these
unwelcome intruders.
~Sources of infection.~ The bacterial life that finds its way into milk
while it is yet on the farm may be traced to several sources, which may
be grouped under the following heads: unclean dairy utensils, fore milk,
coat of animal, and general atmospheric surroundings. The relative
importance of these various factors fluctuates in each individual
instance.
~Dairy utensils.~ Of first importance are the vessels that are used during
milking, and also all storage cans and other dairy utensils that come in
contact with the milk after it is drawn. By unclean utensils, actually
_visible_ dirt need not always be considered, although such material is
often present in cracks and angles of pails and cans. Unless cleansed
with especial care, these are apt to be filled with foul and decomposing
material that suffices to seed thoroughly the milk. Tin utensils are
best. Where made with joints, they should be well flushed with solder so
as to be easily cleaned (see Fig. 6). In much of the cheaper tin ware on
the market, the soldering of joints and seams is very imperfect,
affording a place of refuge for bacteria and dirt.
Cans are often used when they are in a condition wholly unsuitable for
sanitary handling of milk. When the tin coating becomes broken and the
can is rusty, the quality of the milk is often profoundly affected.
Olson[1] of the Wisconsin Station has shown that the action of rennet is
greatly impaired where milk comes in contact with a rusty iron surface.
[Illustration: FIG. 6.]
With the introduction of the form or hand separator a new milk utensil
has been added to those previously in use and one which is very
frequently not well cleaned. Where water is run through the machine to
rinse out the milk particles, gross bacterial contamination occurs and
the use of the machine much increases the germ content of the milk.
Every time the separator is used it should be taken apart and thoroughly
cleaned and dried before reassembling.[2]
~Use of milk-cans for transporting factory by-products.~ The general
custom of using the milk-cans to carry back to the farm the factory
by-products (skim-milk or whey) has much in it that is to be deprecated.
These by-products are generally rich in bacterial life, more especially
where the closest scrutiny is not given to the daily cleaning of the
vats and tanks. Too frequently the cans are not cleaned immediately upon
arrival at the farm, so that the conditions are favorable for rapid
fermentation. Many of the taints that bother factories are directly
traceable to such a cause. A few dirty patrons will thus seriously
infect the whole supply. The responsibility for this defect should,
however, not be laid entirely upon the shoulders of the producer. The
factory operator should see that the refuse material does not accumulate
in the waste vats from day to day and is not transformed into a more or
less putrid mass. A dirty whey tank is not an especially good object
lesson to the patron to keep his cans clean.
It is possible that abnormal fermentations or even contagious diseases
may thus be disseminated.
Suppose there appears in a dairy an infectious milk trouble, such as
bitter milk. This milk is taken to the factory and passes unnoticed into
the general milk-supply. The skim-milk from the separator is of course
infected with the germ, and if conditions favor its growth, the whole
lot soon becomes tainted. If this waste product is returned to the
different patrons in the same cans that are used for the fresh milk, the
probabilities are strongly in favor of some of the cans being
contaminated and thus infecting the milk supply of the patrons. If the
organism is endowed with spores so that it can withstand unfavorable
conditions, this taint may be spread from patron to patron simply
through the infection of the vessels that are used in the transportation
of the by-products. Connell has reported just such a case in a Canadian
cheese factory where an outbreak of slimy milk was traced to infected
whey vats. Typhoid fever among people, foot and mouth disease and
tuberculosis among stock are not infrequently spread in this way. In
Denmark, portions of Germany and some states in America, compulsory
heating of factory by-products is practiced to eliminate this danger.[3]
~Pollution of cans from whey tanks.~ The danger is greater in cheese
factories than in creameries, for whey usually represents a more
advanced stage of fermentation than skim-milk. The higher temperature at
which it is drawn facilitates more rapid bacterial growth, and the
conditions under which it is stored in many factories contribute to the
ease with which fermentative changes can go on in it. Often this
by-product is stored in wooden cisterns or tanks, situated below ground,
where it becomes impossible to clean them out thoroughly. A custom that
is almost universally followed in the Swiss cheese factories, here in
this country, as in Switzerland, is fully as reprehensible as any dairy
custom could well be. In Fig. 7 the arrangement in vogue for the
disposal of the whey is shown. The hot whey is run out through the
trough from the factory into the large trough that is placed over the
row of barrels, as seen in the foreground. Each patron thus has allotted
to him in his individual barrel his portion of the whey, which he is
supposed to remove day by day. No attempt is made to clean out these
receptacles, and the inevitable result is that they become filled with a
foul, malodorous liquid, especially in summer. When such material is
taken home in the same set of cans that is used to bring the fresh milk
(twice a day as is the usual custom in Swiss factories), it is no wonder
that this industry is seriously handicapped by "gassy" fermentations
that injure materially the quality of the product. Not only is the above
danger a very considerable one, but the quality of the factory
by-product for feeding purposes, whether it is skim-milk or whey, is
impaired through the development of fermentative changes.
[Illustration: FIG. 7. Swiss cheese factory (Wisconsin), showing
careless way in which whey is handled. Each patron's share is placed in
a barrel, from which it is removed by him. No attempt is made to cleanse
these receptacles.]
~Improved methods of disposal of by-products.~ The difficulties which
attend the distribution of these factory by-products have led to
different methods of solution. One is to use another separate set of
receptacles to carry back these products to the farm. This method has
been tried, and while it is deemed impracticable by many to handle two
sets of vessels, yet some of the most progressive factories report
excellent results where this method is in use.
Large barrels could be used for this purpose to economize in wagon
space.
Another method that has met with wider acceptance, especially in
creameries, is the custom of pasteurizing or scalding the skim-milk
immediately after it is separated, so that it is returned to the farmer
in a hot condition. In factories where the whole milk is pasteurized,
further treatment of the by-product is not necessary. In most factories
steam, generally exhaust, is used directly in the milk, and experience
has shown that such milk, without any cooling, will keep sweet for a
considerable number of hours longer than the untreated product. It is
noteworthy that the most advanced and progressive factories are the ones
that appreciate the value of this work, and although it involves some
time and expense, experience has shown the utility of the process in
that a better grade of milk is furnished by the patrons of factories
which follow this practice.[4] The exclusion of all danger of animal or
human disease is also possible in this way.
~Cleaning dairy utensils.~ The thorough cleaning of all dairy apparatus
that in any way comes in contact with the milk is one of the most
fundamental and important problems in dairying. All such apparatus
should be so constructed as to permit of easy cleaning. Tinware,
preferably of the pressed variety, gives the best surface for this
purpose and is best suited for the handling of milk.
Milk vessels should never be allowed to become dry when dirty, for dried
particles of milk residue are extremely difficult to remove. In cleaning
dairy utensils they should first be rinsed in lukewarm instead of hot
water, so as to remove organic matter without coagulating the milk. Then
wash thoroughly in hot water, using a good washing powder. The best
washing powders possess considerable disinfecting action.[5] Strong
alkalies should not be used. After washing rinse thoroughly in clean hot
water. If steam is available, as it always is in creameries, cans and
pails should be turned over jet for a few moments. While a momentary
exposure will not suffice to completely sterilize such a vessel, yet
many bacteria are killed in even a short exposure, and the cans dry more
thoroughly and quickly when heated by steam.
Not only should the greatest care be paid to the condition of the cans
and milk-pails, but all dippers, strainers, and other utensils that come
in contact with the milk must be kept equally clean. Cloth strainers,
unless attended to, are objectionable, for the fine mesh of the cloth
retains so much moisture that they become a veritable hot-bed of
bacterial life, unless they are daily boiled or steamed.
The inability to thoroughly render vessels bacteria-free with the
conveniences which are generally to be found on the farm has led in some
cases to the custom of washing and sterilizing the milk cans at the
factory.
~Germ content of milk utensils.~ Naturally the number of bacteria found in
different milk utensils after they have received their regular cleaning
will be subject to great fluctuations; but, nevertheless, such
determinations are of value as giving a scientific foundation for
practical methods of improvement. The following studies may serve to
indicate the relative importance of the utensils as a factor in milk
contamination.
Two cans were taken, one of which had been cleaned in the ordinary way,
while the other was sterilized by steaming. Before milking, the udder
was thoroughly cleaned and special precautions taken to avoid raising of
dust; the fore milk was rejected. Milk drawn into these two cans showed
the following germ content:
No. bacteria Hours before
per cc. souring.
Steamed pail 165 28-1/2
Ordinary pail 4265 23
Harrison[6] has shown how great a variation is in the bacterial content
in milk cans. The utensils were rinsed with 100 cc. of sterile water and
numerical determinations of this rinsing water made. In poorly cleaned
cans, the average germ content was 442,000; in cans washed in tepid
water and then scalded--the best farm practice--54,000, and in cans
carefully washed and then steamed for five minutes, 880.
Another method used by the writer is to wash the utensil with 100 cc.
sterile wash water, using a sterile swab to remove dirt. Then repeat the
process twice or more with fresh rinsing waters, making plate cultures
from each. The following data were obtained from three such
determinations:
No. bacteria in different washings. Total No.
I. II. III. bacteria.
7,800,000 1,450,000 49,000 9,299,000
283,000 43,400 35,000 361,400
1,685,000 105,000 61,400 1,851,400
~Infection of milk in udder cavity.~ A frequently neglected but
considerable factor of infection is that which is attributable to the
bacteria which are present in the udder and which are removed in large
numbers during the milking process. An examination of the fore milk, i.
e., the first few streams from each teat, and that which is subsequently
withdrawn, generally reveals a very much larger number of organisms in
the fore milk.[7] Not infrequently will this part of the milk when drawn
under as careful conditions as possible, contain several score thousand
organisms per cc. If successive bacterial determinations are made at
different periods of the milking, as shown in the following experiment,
a marked diminution is to be noted after the first portion of the milk
is removed:
_Bacterial content at different periods of milking._
Fore 200th 2000th 4300th 6500th Strippings.
milk. cc. cc. cc. cc.
Expt. 1 6,500 1,700 475 220 75 5
Expt. 2 8,100 1,650 400 240 50 10
By some observers it has been claimed that it is possible to secure
absolutely sterile milk in the strippings but this is rarely so. It is
quite probable that such reported results are due to the fact that too
small quantities of milk were used in the examinations and so erroneous
conclusions were drawn. This marked diminution in numbers indicates that
the larger proportion of the organisms found in the fore milk are
present in the lower portion of the udder and milk ducts. When
consideration is given to the structure of the udder, it is readily
apparent that infection will be greater here than above.
[Illustration: FIG. 8. Sectional view of udder showing teat with milk
duct connecting exterior with the milk cistern. Milk sinuses are mostly
shown in cross section interspersed and below the secreting tissue
(Moore and Ward).]
The udder is composed of secreting tissue (_gland cells_) held in place
by fibrous connective tissue. Ramifying throughout this glandular
structure are numerous channels (_milk sinuses_) that serve to convey
the milk from the cells where it is produced into the _milk cistern_, a
common receptacle just above the teats. This cavity is connected with
the exterior through the milk duct in the teat, which is more or less
tightly closed by the circular sphincter muscles, thus preventing the
milk from flowing out. The mucous membranes of the milk duct and cistern
are naturally moist. The habits of the animal render it impossible to
prevent infection of the external opening at the end of the teat and
there is no mechanical reason why bacteria cannot readily find their way
along the moist lining membrane for some distance. If organisms are
adapted to this kind of an environment, ideal conditions exist for their
multiplication, as moisture, warmth and suitable food supply are
present. The question arises how far up into this organ is penetration
possible? Within late years numerous observations have been made on the
presence of organisms in the upper portion of the udder in contact with
the secreting tissue.[8]
These investigations prove that bacteria are distributed throughout the
whole of the udder, although numerically they are much less abundant in
the region of the milk-secreting tissue than in the lower portion.
Ward's conclusions are "that milk when secreted by the glands of a
healthy udder is sterile. It may however, immediately become
contaminated by the bacteria which are normally present in the smaller
milk ducts of the udder."
~Nature of bacteria in fore milk.~ Generally speaking the number of
different species found in the fore milk is not large, and of those
which do appear many occur at only occasional intervals. Moore[9] in the
examination of 9 udders found 20 different forms, and of these only 3
species predominated, all of which proved to be micrococci. Streptococci
have also been quite frequently reported. Freudenreich[10] found the
most common types to be cocci, belonging to both the liquefying and
non-liquefying class.
Peptonizing[11] and spore-bearing[12] species have also been reported as
well as gas-producing[13] forms allied to the colon bacillus. Such
findings are, however, due in all probability to accidental invasion.
Most investigators report the absence of the distinctively lactic-acid
group of organisms.[14]
~Origin of bacteria in udder.~ There is no question but that many of the
types of bacteria found in the udder gain access from the outside. Those
belonging to the spore-bearing, digesting and intestinal types have such
a favorable opportunity for introduction from outside and are so
unlikely to have come directly from the body of the animal, that the
external source of infection is much more probable. Whether this
explanation answers the origin of the cocci that are so generally found
in the upper portion of the udder is questionable. The statement is
ordinarily made that the inner tissues of healthy organs are
bacteria-free, but the studies of Ford[15] seem to indicate that 70 per
cent. of such organs, removed under aseptic conditions from guinea pigs,
rabbits, dogs and cats contained living organisms. Others have reported
similar results in which cocci have been found[16] very similar to those
occurring in the udder. These findings increase the probability that the
origin of this type is from the blood. The persistence of certain
species in the udder for months as noted by Ward indicates possibility
of growth of some forms at least. Stocking[17] has shown where cows are
not milked clean that the germ content of succeeding milkings is greatly
increased.
~Artificial introduction of bacteria into udder.~ If bacteria are capable
of actually developing in the udder proper, it ought to be possible to
easily demonstrate this by the artificial introduction of cultures. In a
number of cases[18] such experiments have been made with various
saprophytic forms, such as _B. prodigiosus_, lactic acid bacilli and
others. In no case has it appeared evident that actual growth has
occurred, although the introduced organism has been demonstrated in
diminishing numbers for 5-6 days. Even the common lactic acid germ and a
yellow liquefying coccus isolated from the fore milk failed to persist
for more than a few days when thus artificially introduced. This failure
to colonize is indeed curious and needs explanation. Is it due to
unsuitable environmental conditions or attributable to the germicidal
influence of the milk?
Various body fluids are known to possess the property of destroying
bacteria and it is claimed by Fokker[19] that this same property was
found in freshly drawn milk. This peculiarity has also been investigated
by Freudenreich,[20] and Hunziker[21] who find a similar property.
No material increase in germ content takes place in milk for several
hours when chilled to 40 deg.-70 deg. F.; on the other hand an actual,
but usually not a marked decrease is observed for about 6 hours. This
phenomenon varies with the milk of different cows. Nothing is known as
to the cause of this apparent germicidal action. The question is yet by
no means satisfactorily settled, although the facts on which the
hypothesis is based are not in controversy. If such a peculiarity
belongs to milk, it is not at all improbable that it may serve to keep
down the germ content in the udder. Freudenreich[22] found that udders
which were not examined for some time after death showed abundant
growth, which fact he attributed to the loss of this germicidal
property.
The infection of the whole milk can be materially reduced by rejecting
the fore milk, but it is questionable whether such rejection is worth
while, except in the case of "sanitary" dairies where milk is produced
with as low a germ content as possible. The intrinsic loss in butter fat
in the fore milk is inconsiderable as the first few streams contain only
about one-fifth the normal fat content.
~Infection of milk after withdrawal from animal.~ The germ content of the
milk, when it is being drawn from the animal is immediately increased
upon contact with the atmosphere. These organisms are derived from the
surrounding air and the utensils in which the milk is received and
stored. The number of organisms which find their way into the milk
depends largely upon the character of the surroundings. Bacteria are so
intimately associated with dirt, dust and filth of all kinds that
wherever the latter are found, the former are sure to be present in
abundance.
The most important factors in the infection of the milk after withdrawal
are the pollution which is directly traceable to the animal herself and
the condition of the milk utensils. Fortunately both of these sources of
contamination are capable of being greatly minimized by more careful
methods of handling.
~Infection directly from the cow.~ It is a popular belief that the
organisms found in milk are derived from the feed and water which the
cow consumes, the same passing directly from the intestinal tract to the
milk by the way of the blood circulation. Such a view has no foundation
in fact as bacteria absorbed into the circulation are practically all
destroyed in the tissues by the action of the body fluids and cells.[23]
While organisms cannot pass readily from the intestine to the udder, yet
this must not be interpreted as indicating that no attention should be
given to the bacterial character of the material consumed. The water
supply given should be pure and wholesome and no decomposed or spoiled
food should be used.
The infection traceable directly to the cow is modified materially by
the conditions under which the animal is kept and the character of the
feed consumed. The nature of the fecal matter is in part dependent upon
the character of the food. The more nitrogenous rations with which
animals are now fed leads to the production of softer fecal discharges
which is more likely to soil the coat of the animal unless care is
taken. The same is true with animals kept on pasture in comparison with
those fed dry fodder.
Stall-fed animals, however, are more likely to have their flanks fouled,
unless special attention is paid to the removal of the manure. All dairy
stalls should be provided with a manure drop which should be cleaned as
frequently as circumstances will permit.
[Illustration: FIG. 9. Showing the bacterial contamination arising from
hair. These hairs were allowed to fall on a sterile gelatin surface. The
adherent bacteria developed readily in this medium, and the number of
bacteria thus introduced into the milk from these hairs can be estimated
by the number of developing colonies.]
The animal herself contributes materially to the quota of germ life
finding its way into the milk through the dislodgment of dust and filth
particles adhering to its hairy coat. The nature of this coat is such as
to favor the retention of these particles. Unless care is taken the
flanks and udder become polluted with fecal matter, which upon drying is
displaced with every movement of the animal. Every hair or dirt particle
so dislodged and finding its way into the milk-pail adds its quota of
organisms to the liquid. This can be readily demonstrated by placing
cow's hairs collected with care on the moist surface of gelatin culture
plates. Almost invariably, bacteria will be found in considerable
numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles
are even richer in germ life. Not only is there the dislodgment of
hairs, epithelial scales and masses of dirt and filth, but during the
milking process, as at all other times, every motion of the animal is
accompanied by a shower of _invisible_ particles more or less teeming
with bacterial life.
The amount of actual impurities found in milk is often considerable and
when it is remembered that about one-half of fresh manure dissolves in
milk,[24] and thus does not appear as sediment, the presence of this
undissolved residue bespeaks filthy conditions as to milking. From
actual tests made, it is computed that the city of Berlin, Germany
consumes about 300 pounds of such dirt and filth daily. Renk has laid
down the following rule with reference to this insoluble dirt: If a
sample of milk shows any evidence of impurity settling on a transparent
bottom within two hours, it should be regarded as too dirty for use.
While the number of organisms here introduced is at all times large, the
character of the species is of even greater import. Derived primarily
from dirt and fecal matter, it is no wonder that such forms are able to
produce very undesirable fermentative changes.
~Influence of milker.~ The condition of the milker is not to be ignored in
determining all possible factors of infection, for when clothed in the
dust-laden garments that have been worn all day, a favorable opportunity
for direct contamination is possible. The filthy practice of wetting the
hands with milk just before milking is to be condemned. The milker's
hands should be washed immediately before milking in clean water and
dried. A pinch of vaseline on hands is sometimes used to obtain a firmer
grasp and prevents the ready dislodgment of scales.[25] It must also be
borne in mind that the milker may spread disease through the milk. In
typhoid fever and diphtheria, the germs often remain in the system for
weeks and thus make infection possible. Stocking[26] has shown that the
individual milker exerts a potent influence on the total germ content of
milk, even where the procedure is quite the same. In sanitary dairies
milkers are usually clad in white duck suits.
~Milking by machinery.~ Several mechanical devices have been invented for
milking, some of which have been tested bacteriologically as to their
efficiency. Harrison[27] has examined the "Thistle" machine but found a
much higher germ content than with hand-drawn milk. The recent
introduction of the Burrel-Lawrence-Kennedy machine has led to numerous
tests in which very satisfactory results have been obtained. If the
rubber parts of the milker are thoroughly cleaned and kept in lime water
solution, they remain nearly sterile. When milk is properly handled, the
germ content may be greatly reduced.
~Reduction in dirt and adherent bacteria.~ No factor of contamination is
so susceptible of improvement as that which relates to the reduction in
filth and dirt which gains access during and immediately subsequent to
the milking. The care which is taken to keep the coat of the animal
clean by grooming lessens very much the grosser portion of such
contamination, but with a dry, hairy coat, fine scales and dust
particles must of necessity be dislodged.[28] Ordinarily the patron
thinks all evidence of such dirt is removed if the milk is strained, but
this process only lessens the difficulty; it does not overcome it.
Various methods are in use, the effectiveness of which is subject to
considerable variation. Some of these look to the elimination of the
bacteria after they are once introduced into the milk; others to the
prevention of infection in the first place.
_1. Straining the milk._ Most of the visible, solid particles of filth,
such as hairs, dirt particles, etc., can be removed by simple straining,
the time-honored process of purification. As ordinarily carried out,
this process often contributes to instead of diminishing the germ life
in milk. The strainer cloths unless washed and thoroughly sterilized by
boiling harbor multitudes of organisms from day to day and may thus
actually add to the organisms present. Various methods have been
suggested for this simple process, but the most practical and efficient
strainer is that made of fine wire gauze to which is added 3-4 layers of
cheese cloth, the whole to set over the storage milk can.
_2. Filtration._ In Europe especially, the system of cleaning milk by
filtration through sand, gravel and other substances has been quite
extensively used. These filters are built in sections and the milk
passes from below upward. The filtering substance is washed in hot water
immediately after use and then steamed and finally baked. While it is
possible to remove the solid impurities in this way, the germ content
cannot be greatly reduced.[29] Cellulose filters have also been
suggested[30] as an improvement over the sand filters. Methods of
filtration of this character have not been used under commercial
conditions here in this country.
_3. Clarification in separator._ Within recent years the custom has
grown of clarifying milk or removing the visible dirt by passing the
milk through a centrifugal separator the cream and skim milk being
remixed after separation. This process naturally removes the solid
impurities as dirt, hairs, epithelial scales and cells, also some of the
casein, making what is known as centrifuge slime. This conglomerate mass
is incomparably rich in germ life and the natural inference would be
that the bacterial content of the milk would be greatly reduced by this
procedure. Eckles and Barnes[31] noted a reduction of 37 to 56 per cent.
of the bacteria but others have failed to observe such reductions.[32]
This condition is explained by the more thorough breaking up of the
bacterial masses in the process, thus apparently not reducing them in
numbers.
It is somewhat surprising that in spite of the elimination of much
organic matter and bacteria, such clarified milk sours as rapidly as the
untreated product.[33]
The mechanical shock of separation ruptures the clusters of fat globules
and so delays creaming and also lessens the consistency of cream derived
from such milk. This practical disadvantage together with the increased
expense of the operation and the failure to materially enhance the
keeping quality of the product outweigh the advantage which might come
from removal of solid impurities which can be largely accomplished on
the farm by efficient straining.
_4. Washing the udder._ If a surface is moist, bacteria adherent to it
cannot be dislodged by ordinary movements. Thus the air over
snow-covered mountains or oceans is relatively devoid of germ life. The
method of moistening the udder is applied with success to the hairy coat
of the animal thus subserving the double purpose of cleaning the animal
and preventing in large measure the continual dislodgment of dust
particles. After these parts have been well carded to remove loose hairs
and dirt particles, the skin should be thoroughly moistened with clean
water and then wiped. It has been urged that this procedure lessens the
yield of milk but Eckles[34] concludes from experiments that when the
animal becomes accustomed to this treatment, no noticeable change in
amount of milk or butter-fat is produced.
The effectiveness of this method in reducing the actual amount of dirt
and filth introduced into the milk as well as the great diminution in
germ life is shown by the instructive experiments of Fraser[35] who
found that the actual amount of dirt dislodged from udders of apparently
clean animals during the milking process was three and one-half times as
much as when the cow's udders were washed. From udders visibly polluted
one ounce of such filth was removed in 275 pounds of milk, while from
cows whose udders had been washed, the same amount of dirt was
distributed through 24,030 pounds.
Fraser observed as a result of 420 examinations that the average germ
content of 4-inch culture dishes under clean but unwashed udders was
578, while under washed animals it was reduced to 192. From numerous
tests made in the writer's laboratory, it is evident that the germ
content of the milk in the pail is increased from 20,000-40,000 bacteria
_per minute_ during the milking period. By far the larger part of this
pollution can be easily prevented by cleaning and dampening the udder.
_5. Diminishing exposed surface of pail._ The entrance of organisms into
the milk can be greatly reduced by lessening the area of the milk pail
directly exposed to the dust shower. A number of so-called sanitary or
hygienic milk pails have been devised for this purpose. In one case the
pail is smaller at the top than bottom, but in most of them the common
form is kept and the exposed area is lessened by means of a cover, the
milk being received through a narrower opening. In some cases, strainers
are also interposed so as to remove more effectually the coarse
particles. It is necessary to have these covers and strainers
constructed in such a way so they can be easily removed and cleaned.
[Illustration: FIG. 10. Sanitary milk pails designed to diminish the
introduction of hairs, scales, dirt, etc., into milk.]
Stocking tested one of these pails (A, Fig. 10) and found that 63 per
cent of the dirt and 29 per cent. of the bacteria were prevented from
passing into the milk. Eckles examined one in which the germ content was
found to be 3200 per cc. as against 43200 per cc. in a common open
pail. This milk did not sour until it was 64 hours old in the first case
while in the latter it curdled in 43 hours.
~Air in barn.~ The atmosphere of the barn where the milking is done may
frequently contribute considerable infection. Germ life is incapable of
development in the air, but in a dried condition, organisms may retain
their vitality for long periods. Anything which contributes to the
production of dust in the stable and aids in the stirring up of the same
increases the number of organisms to be found in the air (Fig. 11).
Thus, the feeding of dry fodder and the bedding of animals with straw
adds greatly to the germ life floating in the air. Dust may vary much in
its germ content depending upon its origin. Fraser found the dust from
corn meal to contain only about one-sixth to one-eighth as much germ
life as that from hay or bran.[36] In time most of these dust particles
settle to the floor, but where the herd is kept in the barn, the
constant movement of the animals keeps these particles more or less in
motion. Much can be done by forethought to lessen the germ content of
stables. Feeding dry feed should not be done until after milking.[37] In
some of the better sanitary dairies, it is customary to have a special
milking room that is arranged with special reference to the elimination
of all dust. In this way this source of infection may be quite obviated
as the air of a clean, still room is relatively free from bacteria,
especially if the floor is moistened. It has often been noted that the
milk of stall-fed animals does not keep as well as that milked out of
doors, a condition in part attributable to the lessened contamination.
[Illustration: FIG. 11. Effect of contaminated air. The number of spots
indicate the colonies that have developed from the bacteria which fell
in 30 seconds on the surface of the gelatin plate (3 inches in
diameter). This exposure was made at time the cows were fed.]
~Relative importance of different sources of infection.~ It is impossible
to measure accurately the influence of the different sources of
infection as these are continually subject to modification in each and
every case. As a general rule, however, where milk is drawn and handled
without any special care, the utensils and the animal contribute the
larger proportion of dirt and bacteria that find their way into milk.
Where the manner of milking and handling is designed to exclude the
largest number of organisms possible, the bacteria appearing in the fore
milk make up the major portion remaining. By putting into practice the
various suggestions that have been made with reference to diminishing
the bacterial content of milk, it is possible to greatly reduce the
number of organisms found therein, and at the same time materially
improve the keeping quality of the milk. Backhaus[38] estimates that
the germ life in milk can be easily reduced to one-two thousandth of its
original number by using care in milking. He reports a series of
experiments covering two years in which milk was secured that averaged
less than 10,000 bacteria per cc., while that secured under ordinary
conditions averaged over 500,000.
[Illustration: FIG. 12. Bacterial content of milk handled in ordinary
way. Each spot represents a colony growing on gelatin plate. Compare
with Fig. 13, where same quantity of milk is used in making culture.
Over 15,000 bacteria per cc. in this milk.]
Fig. 13 gives an illustration as to what care in milking will do in the
way of eliminating bacteria. Fig. 12 shows a gelatin plate seeded with
the same quantity of milk that was used in making the culture indicated
by Fig. 13. The first plate was inoculated with milk drawn under good
conditions, the germ content of which was found to be 15,500 bacteria
per cc., while the sample secured under as nearly aseptic conditions as
possible (Fig. 13) contained only 330 organisms in the same volume.
[Illustration: FIG. 13. Bacterial content of milk drawn with care.
Diminished germ content is shown by smaller number of colonies (330
bacteria per cc.). Compare this culture with that shown in Fig. 12.]
~"Sanitary" or "certified" milk.~ Within recent years there has been more
or less generally introduced into many cities, the custom of supplying
high grade milk that has been handled in a way so as to diminish its
germ content as much as possible. Milk of this character is frequently
known as "sanitary," "hygienic" or "certified," the last term being used
in connection with a certification from veterinary authorities or boards
of health as to the freedom of animals from contagious disease.
Frequently a numerical bacterial standard is exacted as a pre-requisite
to the recommendation of the board of examining physicians. Thus, the
Pediatric Society of Philadelphia requires all children's milk that
receives its recommendation to have not more than 10,000 bacteria per
cc. Such a standard has its value in the scrupulous cleanliness that
must prevail in order to secure these results. This in itself is
practically a guarantee of the absence of those bacteria liable to
produce trouble in children. The number of organisms found in such milks
is surprisingly low when compared with ordinary milk. Naturally, there
is considerable fluctuation from day to day, and occasionally the germ
content is increased to a high figure without any apparent reason. The
average results though, show a greatly reduced number of organisms. De
Schweinitz[39] found in a Washington dairy in 113 examinations extending
throughout a year, an average of 6,485 bacteria per cc. The daily
analyses made of the Walker-Gordon supply sold in Philadelphia for an
entire year, showed that the milk almost always contained less than
5,000 bacteria per cc. and on 120 days out of the year the germ content
was 1,000 organisms per cc. or less.
From a practical point of view, the improvement in quality of sanitary
milk, in comparison with the ordinary product is seen in the enhanced
keeping quality. During the Paris Exposition in 1900, milk and cream
from several such dairies in the United States were shipped to Paris,
arriving in good condition after 15 to 18 days transit. When milk has
been handled in such a way, it is evident that it is much better suited
to serve as a food supply than where it has undergone the fermentative
changes incident to the development of myriads of organisms.
~Application of foregoing precautions to all milk producers.~ Milk is so
susceptible to bacterial changes that it is necessary to protect it from
invasion, if its original purity is to be maintained, and yet, from a
practical point of view, the use to which it is destined has much to do
with the care necessary to take in handling. The effect of the bacterial
contamination of milk depends largely upon the way in which the product
is used. To the milk-man engaged in the distribution of milk for direct
consumption, all bacterial life is more or less of a detriment, while to
the butter-maker and cheese-maker some forms are a direct necessity. It
is unnecessary and impracticable to require the same degree of care in
handling milk destined to be worked up into factory products as is done,
for instance, in sanitary milk supplies, but this fact should not be
interpreted to mean that the care of milk for factories is a matter of
small consequence. In fact no more important dairy problem exists, and
the purer and better the quality of the raw material the better the
product will be. Particularly is this true with reference to
cheese-making.
Dairymen have learned many lessons in the severe school of experience,
but it is earnestly to be hoped that future conditions will not be
summed up in the words of the eminent German dairy scientist, Prof.
Fleischmann, when he says that "all the results of scientific
investigation which have found such great practical application in the
treatment of disease, in disinfection, and in the preservation of
various products, are almost entirely ignored in milking."
~Growth of bacteria in milk.~ Milk is so well suited as a medium for the
development of germ life that it might be expected that all
microorganisms would develop rapidly therein, and yet, as a matter of
fact, growth does not begin at once, even though the milk may be richly
seeded. At ordinary temperatures, such as 70 deg. F., no appreciable
increase is to be noted for a period of 6-9 hours; at lower temperatures
(54 deg. F.) this period is prolonged to 30-40 hours or even longer. After
this period has elapsed, active growth begins and continues more or less
rapidly until after curdling.
The cause of this suspended development is attributed to the germicidal
properties inherent to the milk.[40]
Milk is of course seeded with a considerable variety of organisms at
first. The liquefying and inert species are the most abundant, the
distinctively lactic acid class occurring sparsely, if at all. As milk
increases in age, germ growth begins to occur. More or less development
of all types go on, but soon the lactic species gain the ascendency,
owing to their being better suited to this environment; they soon
outstrip all other species, with the result that normal curdling
generally supervenes. The growth of this type is largely conditioned by
the presence of the milk sugar. If the sugar is removed from milk by
dialysis, the liquid undergoes putrefactive changes due to the fact that
the putrefactive bacteria are able to grow if no acid is produced.
~Relation of temperature to growth.~ When growth does once begin in milk,
the temperature at which it is stored exerts the most profound effect on
the rate of development. When milk is not artificially cooled, it
retains its heat for some hours, and consequently the conditions become
very favorable for the rapid multiplication of the contained organisms,
as is shown in following results obtained by Freudenreich[41]:
_No. of bacteria per cc. in milk kept at different temperatures._
77 deg. F. 95 deg. F.
5 hrs. after milking 10,000 30,000
8 " " " 25,000 12,000,000
12 " " " 46,000 35,280,000
26 " " " 5,700,000 50,000,000
[Illustration: FIG. 14. Effect of cooling milk on the growth of
bacteria.]
Conn[42] is inclined to regard temperature of more significance in
determining the keeping quality than the original infection of the milk
itself. Milk which curdled in 18 hours at 98 deg. F., did not curdle in 48
hours at 70 deg., and often did not change in two weeks, if the
temperature was kept at 50 deg. F.
Where kept for a considerable period at this low temperature, the milk
becomes filled with bacteria of the undesirable putrefactive type, the
lactic group being unable to form acid in any appreciable amounts.
Running well water can be used for cooling, if it is possible to secure
it at a temperature of 48 deg.-50 deg. F. The use of ice, of course, gives
better results, and in summer is greatly to be desired. The influence of
these lowered temperatures makes it possible to ship milk long
distances[43] by rail for city supplies, if the temperature is kept low
during transit.
~Mixing night and morning milk.~ Not infrequently it happens when old milk
is mixed with new, that the course of the fermentative changes is more
rapid than would have been the case if the two milks had been kept
apart. Thus, adding the cooled night milk to the warm morning milk
sometimes produces more rapid changes in both. The explanation for this
often imperfectly understood phenomenon is that germ growth may have
gone on in the cooled milk, and when this material is added to the
warmer, but bacteria-poor, fresh milk, the temperature of the whole mass
is raised to a point suitable for the more rapid growth of all bacteria
than would have occurred if the older milk had been kept chilled.
~Number of bacteria in milk.~ The number of organisms found in milk
depends upon (1) the original amount of contamination, (2) the age of
the milk, and (3) the temperature at which it has been held. These
factors all fluctuate greatly in different cases; consequently, the germ
life is subject to exceedingly wide variations. Here in America, milk
reaches the consumer with less bacteria than in Europe, although it may
often be older. This is due largely to the more wide-spread use of ice
for chilling the milk _en route_ to market. Examinations have been made
of various supplies with the following results: Sedgwick and Batchelder
found in 57 tests of Boston milk from 30,000-4,220,000 per cc. Jordan
and Heineman found 30% of samples of Chicago milk to range from 100,000
to 1,000,000 while nearly one half were from 1-20,000,000 per cc. The
germ content of city milks increase rapidly in the summer months.
Park[44] found 250,000 organisms per cc. in winter, about 1,000,000 in
cool weather and 5,000,000 per cc. in hot summer weather. Knox and
Bassett in Baltimore report 1,500,000 in spring and nearly 4,500,000 in
summer. Eckles[45] studied milk under factory conditions. He finds from
1,000,000 to 5,000,000 per cc. in winter, and in summer from 15-30
millions.
~Bacterial standards for city supplies.~ It would be very desirable to
have a hygienic standard for city milk supplies, as there is a butter
fat and milk-solid test, but the wide spread variation in germ content
and the impracticability of utilizing ordinary bacterial determinations
(on account of time required) makes the selection of such a standard
difficult. Some hold, as Park, that such a standard is feasible. The New
York City Milk commission has set a standard of 30,000 bacteria per cc.
for their certified milk and 100,000 per cc. for inspected milk.
Rochester, N. Y. has attempted the enforcement of such a standard
(limit, 100,000 per cc.) with good results it is claimed while Boston
has placed the legal limit at 500,000 per cc. Quantitative standards
would seem more applicable to "certified" or sanitary supplies than to
general city supplies, where the wide range in conditions lead to such
enormous variations that the bacterial standard seems too refined a
method for practical routine inspection.
~Other tests.~ Any test to be of much service must be capable of being
quickly applied. The writer believes for city milk inspectors that the
acid test would serve a very useful purpose. This test measures the
acidity of the milk. There is, of course, no close and direct
relationship between the development of acidity and the growth of
bacteria, yet in a general way one follows the other at normal
temperatures. Where the temperature is kept rather low, bacterial growth
might go on without much acid development, but in the great majority of
cases a high degree of acidity means either old milk, in which there has
been a long period of incubation, or high temperature, where rapid
bacterial growth has been possible. Either of these conditions
encourages germ growth and thus impairs the quality of the milk.
The rapid determination of acidity may be made in an approximate manner
so as to serve as a test at the weigh-can or intake. The test is best
made by the use of the well known alkaline tablet which is composed of a
solid alkali, and the indicator, phenolphthalein. The tablets are
dissolved in water, one to each ounce used. A number of white cups are
filled with the proper quantity of the solution necessary to neutralize
say, 0.2 per cent. lactic acid. Then, as the milk is delivered, the
proper quantity is taken from each can to which is added the tablet
solution. A retention of the pink color shows that there is not
sufficient acid in the milk to neutralize the alkali used; a
disappearance of color indicates an excess of acid. The standard
selected is of course arbitrarily chosen. In our experience, 0.2 per
cent. acidity (figured as lactic), has proven a satisfactory point. With
carefully handled milk the acidity ought to be reduced to about 0.15 per
cent. The acidity of the milk may be abnormally reduced if milk is kept
in rusty cans, owing to action of acid on the metal.
[Illustration: FIG. 15. Apparatus used in making rapid acid test. A
definite quantity of the alkali solution and indicator is placed in the
white tea cup. To this is added the quantity of milk by means of the
cartridge measure which would just be neutralized if the acidity was 0.2
per cent. A retention of the pink color shows a low acid milk; its
disappearance, a high acid milk.]
~Kinds of bacteria in milk.~ The number of bacteria in milk is not of so
much consequence as the kinds present. With reference to the number of
different species, the more dirt and foreign matter the milk contains,
the larger the number of varieties found in the same. While milk may
contain forms that are injurious to man, still the great majority of
them have no apparent effect on human health. In their effect on milk,
the case is much different. Depending upon their action in milk, they
may be grouped into three classes:
1. Inert group, those producing no visible change in the milk.
2. Sour milk forms, those breaking up the milk sugar with or without the
formation of gas.
3. Digesting or peptonizing group, those capable of rendering the casein
of milk soluble and more or less completely digested.
A surprisingly large number of bacteria that are found in milk belong to
the first class. Undoubtedly they affect the chemical characteristics of
the milk somewhat, but not to the extent that it becomes physically
perceptible. Eckles[46] reports in a creamery supply from 20 to 55 per
cent. of entire flora as included in this class.
By far the most important group is that embraced under the second head.
It includes not only the true lactic acid types in which no gas is
formed, but those species capable of producing gases and various kinds
of acids. These organisms are the distinctively milk bacteria, although
they do not predominate when the milk is first drawn. Their adaptation
to this medium is normally shown, however, by this extremely rapid
growth, in which they soon gain the ascendency over all other species
present. It is to this lactic acid class that the favorable
flavor-producing organisms belong which are concerned in butter-making.
They are also indispensable in cheese-making.
The third class represents those capable of producing a liquefied or
digested condition on gelatin or in milk. They are usually the
spore-bearing species which gain access from filth and dirt. Their high
powers of resistance due to spores makes it difficult to eradicate this
type, although they are materially held in subjection by the lactic
bacteria. The number of different kinds that have been found in milk is
quite considerable, something over 200 species having been described
more or less thoroughly. In all probability, however, many of these
forms will be found to be identical when they are subjected to a more
critical study.
~Direct absorption of taints.~ A tainted condition in milk may result from
the development of bacteria, acting upon various constituents of the
milk, and transforming these in such a way as to produce by-products
that impair the flavor or appearance of the liquid; or it may be
produced by the milk being brought in contact with any odoriferous or
aromatic substance, under conditions that permit of the direct
absorption of such odors.
This latter class of taints is entirely independent of bacterial action,
and is largely attributable to the physical property which milk
possesses of being able to absorb volatile odors, the fat in particular,
having a great affinity for many of these substances. This direct
absorption may occur before the milk is withdrawn from the animal, or
afterwards if exposed to strong odors.
It is not uncommon for the milk of animals advanced in lactation to have
a more or less strongly marked odor and taste; sometimes this is apt to
be bitter, at other times salty to the taste. It is a defect that is
peculiar to individual animals and is liable to recur at approximately
the same period in lactation.
The peculiar "cowy" or "animal odor" of fresh milk is an inherent
peculiarity that is due to the direct absorption of volatile elements
from the animal herself. This condition is very much exaggerated when
the animal consumes strong-flavored substances as garlic, leeks, turnips
and cabbage. The volatile substances that give to these vegetables their
characteristic odor are quickly diffused through the system, and if such
foods are consumed some few hours before milking, the odor in the milk
will be most pronounced. The intensity of such taints is diminished
greatly and often wholly disappears, if the milking is not done for some
hours (8-12) after such foods are consumed.
This same principle applies in lesser degree to many green fodders that
are more suitable as feed for animals, as silage, green rye, rape, etc.
Not infrequently, such fodders as these produce so strong a taint in
milk as to render it useless for human use. Troubles from such sources
could be entirely obviated by feeding limited quantities of such
material immediately after milking. Under such conditions the taint
produced is usually eliminated before the next milking. The milk of
swill-fed cows is said to possess a peculiar taste, and the urine of
animals fed on this food is said to be abnormally acid. Brewers' grains
and distillery slops when fed in excess also induce a similar condition
in the milk.
Milk may also acquire other than volatile substances directly from the
animal, as in cases where drugs, as belladonna, castor oil, sulfur,
turpentine, jalap, croton oil, and many others have been used as
medicine. Such mineral poisons as arsenic have been known to appear
eight hours after ingestion, and persist for a period of three weeks
before being eliminated.
~Absorption of odors after milking.~ If milk is brought in contact with
strong odors after being drawn from the animal, it will absorb them
readily, as in the barn, where frequently it is exposed to the odor of
manure and other fermenting organic matter.
It has long been a popular belief that milk evolves odors and cannot
absorb them so long as it is warmer than the surrounding air, but from
experimental evidence, the writer[47] has definitely shown that the
direct absorption of odors takes place much more rapidly when the milk
is warm than when cold, although under either condition, it absorbs
volatile substances with considerable avidity. In this test fresh milk
was exposed to an atmosphere impregnated with odors of various essential
oils and other odor-bearing substances. Under these conditions, the
cooler milk was tainted very much less than the milk at body temperature
even where the period of exposure was brief. It is therefore evident
that an exposure in the cow barn where the volatile emanations from the
animals themselves and their excreta taint the air will often result in
the absorption of these odors by the milk to such an extent as to
seriously affect the flavor.
The custom of straining the milk in the barn has long been deprecated as
inconsistent with proper dairy practice, and in the light of the above
experiments, an additional reason is evident why this should not be
done.
Even after milk is thoroughly cooled, it may absorb odors as seen where
the same is stored in a refrigerator with certain fruits, meats, fish,
etc.
~Distinguishing bacterial from non-bacterial taints.~ In perfectly fresh
milk, it is relatively easy to distinguish between taints caused by the
growth of bacteria and those attributable to direct absorption.
If the taint is evident at time of milking, it is in all probability due
to character of feed consumed, or possibly to medicines. If, however,
the intensity of the taint grows more pronounced as the milk becomes
older, then it is probably due to living organisms, which require a
certain period of incubation before their fermentative properties are
most evident.
Moreover, if the difficulty is of bacterial origin, it can be frequently
transferred to another lot of milk (heated or sterilized is preferable)
by inoculating same with some of the original milk. Not all abnormal
fermentations are able though to compete with the lactic acid bacteria,
and hence outbreaks of this sort soon die out by the re-establishment of
more normal conditions.
~Treatment of directly absorbed taints.~ Much can be done to overcome
taints of this nature by exercising greater care in regard to the feed
of animals, and especially as to the time of feeding and milking. But
with milk already tainted, it is often possible to materially improve
its condition. Thorough aeration has been frequently recommended, but
most satisfactory results have been obtained where a combined process of
aeration and pasteurization was resorted to. Where the milk is used in
making butter, the difficulty has been successfully met by washing the
cream with twice its volume of hot water in which a little saltpeter has
been dissolved (one teaspoonful per gallon), and then separating it
again.[48]
The treatment of abnormal conditions due to bacteria has been given
already under the respective sources of infection, and is also still
further amplified in following chapter.
~Aeration.~ It is a common belief that aeration is of great aid in
improving the quality of milk, yet, when closely studied, no material
improvement can be determined, either where the milk is made into butter
or sold as milk. Dean in Canada and Storch in Denmark have both
experimented on the influence of aeration in butter making, but with
negative results. Marshall and Doane failed to observe any material
improvement in keeping quality, but it is true that odors are eliminated
from the milk during aeration. The infection of the milk during aeration
often more than counterbalances the reputed advantage. Especially is
this so if the aeration is carried out in an atmosphere that is not
perfectly clean and pure.
In practice aeration differs greatly. In some cases, air is forced into
the milk; in others, the milk is allowed to distribute itself in a thin
sheet over a broad surface and fall some distance so that it is brought
intimately in contact with the air. This latter process is certainly
much more effective if carried out under conditions which preclude
infection. It must be remembered that aeration is frequently combined
with cooling, in which case the reputed advantages may not be entirely
attributable to the process of aeration.
~Infection of milk in the factory.~ The problem of proper handling of milk
is not entirely solved when the milk is delivered to the factory or
creamery, although it might be said that the danger of infection is much
greater while the milk is on the farm.
In the factory, infection can be minimized because effective measures of
cleanliness can be more easily applied. Steam is available in most
cases, so that all vats, cans, churns and pails can be thoroughly
scalded. Special emphasis should be given to the matter of cleaning
pumps and pipes. The difficulty of keeping these utensils clean often
leads to neglect and subsequent infection. In Swiss cheese factories,
the custom of using home-made rennet solutions is responsible for
considerable factory infection. Natural rennets are soaked in whey which
is kept warm in order to extract the rennet ferment. This solution when
used for curdling the milk often adds undesirable yeasts and other
gas-generating organisms, which are later the cause of abnormal ferment
action in the cheese (See page 186).
The influence of the air on the germ content of the milk is, as a rule,
overestimated. If the air is quiet, and free from dust, the amount of
germ life in the same is not relatively large. In a creamery or factory,
infection from this source ought to be much reduced, for the reason that
the floors and wall are, as a rule, quite damp, and hence germ life
cannot easily be dislodged. The majority of organisms found under such
conditions come from the person of the operators and attendants. Any
infection can easily be prevented by having the ripening cream-vats
covered with a canvas cloth. The clothing of the operator should be
different from the ordinary wearing-apparel. If made of white duck, the
presence of dirt is more quickly recognized, and greater care will
therefore be taken than if ordinary clothes are worn.
The surroundings of the factory have much to do with the danger of germ
infection. Many factories are poorly constructed and the drainage is
poor, so that filth and slime collect about and especially under the
factory. The emanations from these give the peculiar "factory odor" that
indicates fermenting matter. Not only are these odors absorbed
directly, but germ life from the same is apt to find its way into the
milk. Connell[49] has recently reported a serious defect in cheese that
was traced to germ infection from defective factory drains.
The water supply of a factory is also a question of prime importance.
When taken from a shallow well, especially if surface drainage from the
factory is possible, the water may be contaminated to such an extent as
to introduce undesirable bacteria in such numbers that the normal course
of fermentation may be changed. The quality of the water, aside from
flavor, can be best determined by making a curd test (p. 76) which is
done by adding some of the water to boiled milk and incubating the same.
If "gassy" fermentations occur, it signifies an abnormal condition. In
deep wells, pumped as thoroughly as is generally the case with factory
wells, the germ content should be very low, ranging from a few score to
a few hundred bacteria per cc. at most.
Harrison[50] has recently traced an off-flavor in cheese in a Canadian
factory to an infection arising from the water-supply. He found the same
germ in both water and cheese and by inoculating a culture into
pasteurized milk succeeded in producing the undesirable flavor. The
danger from ice is much less, for the reason that good dairy practice
does not sanction using ice directly in contact with milk or cream.
Then, too, ice is largely purified in the process of freezing, although
if secured from a polluted source, reliance should not be placed in the
method of purification; for even freezing does not destroy all
vegetating bacteria.
FOOTNOTES:
[1] Olson. 24 Rept. Wis. Expt. Stat., 1907.
[2] Erf and Melick Bull. 131, Kan. Expt. Stat., Apr. 1905.
[3] Storch (40 Rept. Danish Expt. Stat., Copenhagen, 1898) has devised a
test whereby it can be determined whether this treatment has been
carried out or not: Milk contains a soluble enzym known as galactase
which has the property of decomposing hydrogen peroxid. If milk is
heated to 176 deg. F. (80 deg. C.) or above, this enzym is destroyed so
that the above reaction no longer takes place. If potassium iodid and
starch are added to unheated milk and the same treated with hydrogen
peroxid, the decomposition of the latter agent releases oxygen which
acts on the potassium salt, which in turn gives off free iodine that
turns the starch blue.
[4] McKay, N. Y. Prod. Rev., Mch. 22, 1899.
[5] Doane, Bull. 79, Md. Expt. Stat., Jan. 1902.
[6] Harrison, 22 Rept. Ont. Agr'l Coll., 1896, p. 113.
[7] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899; Ward,
Bull. 178, Cornell Expt. Stat., Jan. 1900.
[8] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108; Moore, 12 Rept.
Bur. Animal Ind., U. S. Dept. Ag., 1895-6, p. 261.
[9] Moore, Bacteria in Milk, N. Y. Dept. Ag., 1902.
[10] Freudenreich, Cent. f. Bakt., II Abt., 10: 418, 1903.
[11] Harrison, 22 Rept. Ont. Agr. Coll., 1896, p. 108.
[12] Marshall, Bull. 147, Mich. Expt. Stat., p. 42.
[13] Moore and Ward, Bull. 158, Cornell Expt. Stat., Jan. 1899.
[14] Burr, R. H. Cent. f. Bakt., II Abt., 8: 236, 1902. Freudenreich, l.
c. p. 418. Ward, Bull. 178, Cornell Expt. Stat., p. 277. Bolley (Cent.
f. Bakt., II Abt., 1: 795, 1895), in 30 experiments found 12 out of 16
species to belong to lactic class. Harrison (Trans. Can. Inst., 7: 474,
1902-3) records the lactic type as most commonly present.
[15] Ford, Journ. of Hyg., 1901, 1: 277.
[16] Freudenreich, l. c. p. 421.
[17] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
[18] Dinwiddie, Bull, 45 Ark. Expt. Stat., p. 57. Ward, Journ. Appld.
Mic. 1: 205, 1898. Appel, Milch Zeit., No. 17, 1900. Harrison and
Cumming, Journ. Appld. Mic. 5: 2087. Russell and Hastings, 21 Rept. Wis.
Expt. Stat., 158, 1904.
[19] Fokker, Zeit. f. Hyg., 9: 41, 1890.
[20] Freudenreich, Ann. de Microg., 3: 118, 1891.
[21] Hunziker, Bull. 197, Cornell Expt. Stat., Dec. 1901.
[22] Freudenreich, Cent. f. Bakt., II Abt., 10: 417, 1903.
[23] This general statement is in the main correct, although Ford
(Journ. of Hyg., 1: 277, 1901) claims to have found organisms sparingly
present in healthy tissues.
[24] Backhaus, Milch Zeit., 26: 357, 1897.
[25] Freudenreich, Die Bakteriologie, p. 30.
[26] Stocking, Bull. 42, Storrs Expt. Stat., June 1906.
[27] Harrison, Cent. f. Bakt., II Abt., 5: 183, 1899.
[28] Drysdale, Trans. High. and Agr. Soc. Scotland. 5 Series, 10: 166,
1898.
[29] Schuppan, (Cent. f. Bakt., 13: 155, 1893) claims to have found a
reduction of 48 per cent. in the Copenhagen filters while in the more
extended work of Dunbar and Kister (Milch Zeit., pp. 753, 787, 1899) the
bacterial content was higher in the filtered milk in 17 cases out of 22.
[30] Backhaus and Cronheim, Journ. f. Landw., 45: 222, 1897.
[31] Eckles and Barnes, Bull. 159 Iowa Expt. Stat., Aug. 1901.
[32] Dunbar and Kister, Milch Zeit., p. 753, 1899. Harrison and Streit,
Trans. Can. Inst., 7: 488, 1902-3.
[33] Doane, Bull. 88 Md. Expt. Stat., May 1903.
[34] Eckles, Hoard's Dairyman, July 8, 1898.
[35] Fraser, Bull. 91, Ill. Expt. Stat.
[36] Fraser, Bull. 91, Ill. Expt. Stat., Dec. 1903.
[37] Stocking, Bull. 42, Storrs Expt. Stat., June, 1906.
[38] Backhaus. Ber. Landw. Inst. Univ. Koenigsberg 2: 12, 1897.
[39] De Schweinitz, Nat. Med. Rev., April, 1899.
[40] Conn, Proc. Soc. Amer. Bacteriologists, 1902.
[41] Freudenreich, Ann. de Microg., 2:115, 1890.
[42] Conn, Bull. 26, Storrs Expt. Stat.
[43] New York City is supplied with milk that is shipped 350 miles.
[44] Park, N. Y. Univ. Bull., 1: 85, 1901.
[45] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
[46] Eckles, Bull. 59, Iowa Expt. Stat., Aug. 1901.
[47] Russell, 15 Rept. Wis. Expt. Stat. 1898, p. 104.
[48] Alvord, Circ. No. 9, U. S. Dept. Agric. (Div. of Bot.).
[49] Connell, Rept. of Commissioner of Agr., Canada, 1897, part XVI, p.
15.
[50] Harrison, Hoard's Dairyman, March 4, 1898.